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What they do
GPS receivers provide highly accurate worldwide position fixes, 24 hours a day, in all
weather. For many navigators, simply knowing position is sufficient. But the real power of
GPS receivers is their ability to direct you to a destination. After you enter a waypoint,
your GPS will tell you the distance and direction to it, your off-course error, and the
time left until you reach the waypoint. Unlike Loran C, GPS provides very accurate speed
and course information with short averaging times.
How they work
GPS receivers receive signals from high-altitude satellites operated by the U.S. Air
Force. By timing the signals sent by the satellites, and by knowing the orbital parameters
of the satellites, a GPS receiver can determine your location to an accuracy within 6
meters, 95% of the time, and your altitude within approximately 10 meters. (More on
accuracy later in this Advisor.)
Unlike Loran C, which has an unsettling tendency to provide "bad fixes" under
certain conditions, the GPS system is highly resistant to giving false information. In
fact, the majority of GPS receivers indicate the accuracy of their fixes in feet or
meters, which varies as satellite positions change. What to Look for
Receiver Type
Virtually all of the GPS models we sell have 12 channel receivers, which have largely
eliminated the need to specify the type of receiver you are looking for. Even sub-$200
models provide excellent time to first fix, and work well in areas with obstructions, such
as forests and canyons (urban as well as geographic.) What is probably of greater interest
is the antenna to which the receiver is connected. While it is generally possible to get a
fix while operating a GPS on your lap while driving (not recommended!) or while below
decks on a fiberglass boat, your best results will be had when the antenna has an
uninterrupted view of the sky. A small external antenna or an extension cable on a
handheld's antenna, can make a dramatic difference in fix quality.
Display
All but a few GPS receivers have graphic LCD displays. While alphanumeric displays still
provide useful data, graphics are a real plus. They can display steering information, plot
where you have been, create more legible characters, and show cartographic information.
Display legibility is a key variable among GPS models. LCDs vary greatly in contrast, and
the use of polarized screens causes sunglass wearers to have to remove them when viewing.
The "blackness" of the characters is important. Some high-resolution displays
have very thin characters (we refer to them as the Wedding Invitation Font) that wash out
in normal lighting conditions. Bottom line: poor legibility negates any clever features a
GPS may have.
Cartographic Displays
We like GPS displays that show your position superimposed on an electronic chart or map.
While traditionalists may scoff at trusting a digital map, we think that seeing your GPS
position on a high-quality electronic chart is a giant convenience and provides
dramatically increased safety. Waypoint and route storage is fast and relatively
error-free, since data entry errors are practically eliminated.
For fixed-mount GPS, we find it difficult to recommend a GPS that doesn't have the ability
to display charts, since prices begin in the $400 range and they are so much more useful
than plotter-only models. Even if you don't plan to use digital charts right now, or you
are traveling to areas for which charts aren't available, they still have great displays,
user-interfaces and increased capabilities.
The newest cartographic GPS have excellent built-in map databases, so you may not have to
buy additional (expensive) chart cartridges. While the maps are land-oriented, and have
less detail than those found on chart cartridges, we find the data to be extremely useful
and the products an excellent value.
For those who need the portability of a handheld GPS, but don't want to give up having a
cartographic receiver, there are now several excellent handhelds. Best, in our opinion,
are the Garmin GPS III Plus and eTrex Legend, and we look forward to learning more about
the new Magellan 330 and Lowrance iFinder. We like them for their detailed displays and
great user-interfaces. The GPS III Plus and the eMap can accept additional data from
CD-ROMs, as can the Magellan and Lowrance models. A single CD-ROM database holds the
equivalent of hundreds of paper maps. This information can be downloaded to your GPS's
memory or onto memory cartridges, depending on the unit. When you no longer need the data
you've downloaded, simply write over it by downloading data for a new area.
Garmin's GPS III Plus and eTrex Legend/Vista have internal flash memory, while the eMap
and StreetPilots use a flash memory cartridge which stores data from the CD-ROM. For the
type of map data you need, choose from five Garmin MapSource CD-ROM titles. They are US
Metroguide, US Waterways, US Roads and Recreation, US Topographic and WorldMap.
Unfortunately for boaters, none of these CDs has what we would consider a
navigation-quality marine database, although the new US Waterways is pretty
complete.
Ease of Use
We think that a single screen should display all the information you need to fetch a
destination, without requiring you to toggle between multiple screens. The best approach
is a user-modifiable navigation screen that can display whatever functions you find
helpful. We like range/bearing, heading error, and time-to-go, but you will have your own
preferences. If you navigate in channels, for example, you might prefer a graphic steering
indicator, crosstrack error and course-to-steer to help you stay in the center of the
channel. Each screen or page should have a purpose. Don't be misled by the number of
screens-it is the utility and clarity of the presentation that matters.
Accuracy, WAAS, Differential, and Selective Availability
A tremendous amount occurred in 2000 to change the accuracy of GPS. Prior to May, 2000,
GPS signals were degraded (Selective Availability or S/A) from their inherent accuracy of
15m 95% of the time, to 100m 95% of the time, ostensibly to deprive terrorists and other
bad guys from the advantages of having a really accurate positioning system for launching
missiles and other hostile acts. However, in May of 2000, President Clinton lifted S/A,
which increased GPS accuracy to 15m (most users reported that it was actually 10 meters or
better). Suddenly, every GPS in the world became dramatically more accurate and useful.
Differential GPS was developed to counteract the errors imposed by S/A by utilizing a low
frequency signal containing GPS error correction messages. DGPS results in accuracies of
about 6m, whenever the user is located within about 200 miles of a station. To achieve
this accuracy you must usually connect a separate Differential Beacon Receiver (DBR) to
the GPS, though some units now have a DBR built in. This receiver decodes low-frequency
signals in the 300kHz range sent from Coast Guard and Army Corps of Engineers beacon
transmitters.
However, the real news occurred in the fourth quarter of 2000 when the first WAAS-capable
GPS receivers became available from Raytheon. WAAS is a system that uses satellites to
transmit GPS corrections virtually worldwide, using Inmarsat geostationary satellites.
Since WAAS corrections are based on many land station measurements, it results in
ionospheric corrections that increase the ultimate accuracy to less than 3m (10'). In
addition, the WAAS corrections occur within the GPS using sophisticated software, so there
is no secondary receiver or antenna to buy.
Many recreational boaters will ask (and rightly so) whether they need any incremental
accuracy over the current non-S/A signals. We feel that the vast majority of users are
well served by 10m accuracy, and will not gain obvious advantages from DGPS or WAAS.
Remember that speed and heading accuracy are also improved when positional accuracy is
improved (and so is altitude measurement.)
However, if we could purchase a WAAS capable receiver at a modest up charge compared to a
non-WAAS receiver, we'd buy it in a heartbeat.
Weather Resistance
The term "waterproof" means different things to different GPS makers. Almost all
GPS models can withstand intense spray, rain, wet hands and other normal sources of
moisture. For ultimate reliability, completely sealed cases work best, since not even
humidity can enter the case. Make sure that the manufacturer covers water intrusion in the
warranty-if not, keep your GPS well protected in a waterproof bag (or below decks).
Moreover, obviously, don't depend on one little GPS that costs $150 as the sole means of
navigation for a $100,000 yacht! Navigators use a variety of methods to ensure the safety
of their crew and craft.
Waypoint and Route Storage Capabilities
The GPS units we offer can store from 100 to 1,000 locations, or waypoints. Most receivers
allow you to name a waypoint with a 4-16 character (alphanumeric) name. We really like
this feature and would not consider a GPS with number-only waypoint designations.
The ability to store several routes, or strings of waypoints, is also a very useful
feature. Most GPS model store 10-20 routes, with 10-30 waypoints in each route. Reversible
routes can be traversed in either direction, which effectively doubles the number of
routes in a GPS.
Alarms
Alarms can be a valuable safety feature to alert you to a hazardous condition or
position-when you are seriously off course, when you pass within a certain distance of a
waypoint, or when you travel more than a preset distance away from a waypoint or spot
where you anchored. All alarms have their uses; none are critical.
Conclusions
In our opinion, GPS receivers are one of the best examples of high-value consumer
products. For as little as $100, you can know your position in any weather, anywhere on
earth, with great accuracy. Boaters can find destinations with greater safety, fishermen
can catch more fish, hikers can find their way in forests, and UPS and Fed Ex trucks can
find obscure addresses in rural areas. Like the revolution in personal computers, which
began in the early 1980s, the GPS revolution is a great example of increasing
functionality, increasing ease of use, and decreasing costs. Okay, it did cost $14 billion
to create the system and launch the satellites, but now we get to use the satellites
essentially free.
At safety seminars we are always quick to point out that one of the most critical elements
of marine safety is to know where you are. With GPS, and local charts, knowing where you
are is only a few keystrokes away. And that, is a tremendous safety advantage.
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E-TREX GPS RECEIVER
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e Trex Mariner
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FlusFlush Mount Kit
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FlushFlSerial Data Cable
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